A Need to Discover the World of Giant Viruses

tion of microbes [2] . As a matter of fact, in the 20th century, some giant viruses were considered for a long time to be bacteria or parasites because the size factor biased their initial viral classification. Indeed, the established definition of a virus during the 20th century considered that all viruses should have at least one dimension of less than 0.2 μm [2, 7] . Giant viruses pose a problem to traditional virology techniques because purification techniques prior to virome study involve a filtration that removes giant viruses [12–14] . We were recently able to show that, when avoiding 0.2-μm filtration, giant viruses belonging to the Marseillevirus family could be found circulating in the blood of asymptomatic patients [13, 14] . In addition, for now, PCR techniques have unfortunately demonstrated little sensitivity because, due to the high variability of these viruses, tools were lacking to identify them [2, 15] . Culture techniques have indeed enabled the isolation of these viruses for the first time in human stool samples, with a reference reported in this paper [16] . New strategies for culture, purification [17] and isolation techniques [16] are reported in the present issue. Also, for the first time, these techniques have enabled the isolation of Mimivirus from a respiratory sample [18, 19] and the stools of a patient with pneumonia [20] . However, it is likely that serology is more effective in the detection of specific antibodies against the Marseilleviridae family, as related in Gilbert Greub’s work in this journal [21] . These data confirm that both Mimiviridae and Marseilleviridae may be found in humans [19] . Mimivirus The world of giant viruses was brutally opened up at the beginning of the 21st century when a microorganism visible under the microscope, Gram stained, and which had been considered a bacterium for several years, was identified as the largest virus known at the time: it was Mimivirus [1, 2] . Its genome contained more than 1,200 genes and had the particularity, given the wealth of information it contained, to harbor genes common to bacteria, archaea and eukaryotes [3] . To me, this particular feature was the most striking, and I postulated immediately for the publication of the genome, believing that this virus might be representative of a fourth domain [3] . This caused various reactions, in spite of a subsequent classification into four branches using genes encoding for RNA polymerase showing four groups: giant viruses, archaea, bacteria and eukaryotes [4, 5] . For a while, I was the only one to believe in the existence of a fourth domain, which was challenged both by Jean-Michel Claverie and C.E. Woese [6] . We proposed later, based on these data, to create a new classification splitting organisms into capsidencoding organisms and ribosome-encoding organisms [7] . In this issue, the specificity of codon and amino acid use of this group of viruses is reported [8] . Since then, we have reported the existence of a new family of viruses, Marseilleviridae [9] , the first strain being Marseillevirus [10] . The recent discovery of a new giant virus, considered to be an intracellular eukaryotic parasite for 15 years [11] , shows that the world of giant viruses keeps expanding and challenges the traditional classificaPublished online: October 17, 2013